Read another document from the Ústí nad Labem StB, preserved in the Security Forces Archive:
Attention!
The upcoming Jewish New Year (around September 20th) will probably be used by Jews, or rather Zionists, to send New Year's greetings by possibly adding various notes, also in Hebrew, often of a confidential nature. This is a fairly inconspicuous way to send messages to the remaining Zionists here - of course, checking the letters would be very difficult and at most we would be able to determine the addressees and senders of these letters, which would require the participation of all offices in the republic, which would be very limited from a technical point of view (it would take a lot of time). However, based on the addresses, a fairly good record of the remaining Zionists here could be obtained.
Why do you think the Czechoslovak State Security believed that Jewish New Year's cards contained secret encrypted messages?